In his Private Law Sources and Analogies of International Law (1927) Hersch Lauterpacht claimed that many rules and concepts of international law stemmed from private law. He also showed that it was common practice in international adjudication and arbitration to look for inspiration there. The rules of private law that had found their way to international law were often common to the great municipal law systems. Many had their origins in Roman private law.

This article examines whether and how the ICJ has made use of Roman law rules and concepts. Roman law can be thought to fulfil its role as a source of inspiration for international law in three ways. First, it might have served as a direct historical source during the formative period of the modern law of nations. Second, it might have served as an indirect historical source because of its enduring impact on the great municipal law systems afterwards. Thirdly, it might still be considered ratio scripta, the expression of a timeless and universal law. For the purpose of examining which of these roles Roman law plays in the eyes of the ICJ, the analysis is restricted to two examples of private law analogies: occupation of terra nullius and acquisitive prescription.